The Motion Pictures Expert Group (MPEG) 4 advanced video coding (AVC) standard specifies the use of long-term reference frames (LTRFs). LTRFs are designed to have a greater persistence than traditional MPEG-2 I-frames and can be a designated B-frame, P-frame, or any other temporally predictive INTER frame requiring less bandwidth than an I-frame (or other fully independently coded frame). I-frames usually persist for the duration of a group of pictures (GOPs) and can also be used as a reference frame for error correction when errors occur over the transmission network. On the other hand, LTRFs can persist indefinitely and can also be used be used as a reference frame when errors occur over the network. The MPEG-4 standard defines a number of slots or placeholders in memory for LTRFs. LTRFs are generated by the encoder and identified in the encoded video stream. Both the encoder and decoder store the LTRFs.
When using LTRFs error feedback from the decoder is desired because any error in a frame (LTRF or not) will continue to propagate over time, i.e., future frames will continue to reference (or motion predict from) bad data in another decoded frame. The decoder can inform the encoder of errors such as a lost or damaged packet. An encoder receiving such feedback can correct the error, e.g., by encoding the next frame in such a way that it does not reference the erroneous data. One method to correct errors is to reference an LTRF that has been confirmed by the decoder not to contain errors and thereby avoid the need to send a higher bandwidth I-frame. In this case, the encoder needs to indicate to the decoder which LTRF is the reference frame for predictive coding. Error feedback also ensures that the LTRFs stored at the encoder and decoder are the same.